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co  p.  2, 


AGRICULTURE 


NON  CIRCULATING 

CHECK  FOR  UNBOUND 
CIRCULATING  COPY 


UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  298 


WINTER  FORCING  OF  RHUBARB 


BY  C.  B.  SAYRB 


URBANA,  ILLINOIS,  OCTOBER,  1927 


SUMMARY 

In  commercial  rhubarb  forcing  heavy  yields  of  dark  pink  stalks 
are  desired.  There  is  considerable  variation  in  commercial  practice  and 
in  opinions  as  to  the  best  methods  of  obtaining  these  results. 

In  these  experiments  roots  were  forced  in  controlled  temperature 
chambers  and  it  was  found  that  the  lower  the  forcing  temperature  the 
darker  red  were  the  stalks  and  the  slower  the  growth.  At  59°  F.  max- 
imum yields  of  well-colored  stalks  were  obtained.  An  earlier  crop  of 
inferior  color  and  lighter  yield  was  produced  at  68°  F.  Slow  growing, 
dark  red  stalks  and  a  light  yield  were  produced  at  50°  F.  No  market- 
able stalks  were  obtained  at  77°  F.  or  above. 

Forced  rhubarb  is  over  95  percent  water,  yet  many  commercial 
growers  do  not  water  the  roots  during  the  forcing  period  for  fear  of 
"washing  out  the  color."  In  these  experiments  watering  increased  the 
yields  three-fold  and  did  not  "wash  out"  the  color. 

Roots  of  the  same  strain  but  different  ages  (one,  two,  three,  five, 
and  six  years  old)  were  forced  under  similar  conditions.  The  one-year- 
old  roots  produced  lighter  colored  stalks  and  smaller  yields  than  the 
older  roots.  There  was  no  appreciable  difference  in  color  of  stalks  pro- 
duced by  the  two-,  three-,  five-,  and  six-year-old  roots.  Yields  in- 
creased with  the  age  of  the  roots  up  to  three  years.  Above  that  age 
they  were  practically  constant. 

Some  roots  that  had  not  been  frozen  were  placed  in  the  forcing 
cellar  under  the  same  conditions  as  similar  roots  that  had  been  frozen. 
A  prompt  growth  resulted  from  the  latter,  but  there  was  no  growth  on 
the  unfrozen  roots.  Later  these  were  frozen  and  stimulated  to  growth. 

To  determine  the  effect  of  different  freezing  temperatures,  roots 
were  placed  in  cold  storage  chambers  at  25°,  20°,  10°,  and  —  10°  F. 
and  later  were  forced  under  similar  conditions.  The  roots  did  not  freeze 
at  25°  F.,  but  at  20°  they  were  frozen  thoroly.  Light  freezing  stimu- 
lated growth,  but  severe  freezing  was  injurious  and  reduced  the  yield. 
Brief  thoro  freezing  followed  by  a  dormant  period  was  as  effective  in 
stimulating  growth  as  was  freezing  thruout  the  dormant  period. 

Soon  after  digging  in  the  fall,  roots  of  various  ages  were  frozen 
for  two  weeks  in  cold  storage  chambers  at  20°,  10°,  and  —10°  F. 
Half  of  each  lot  of  roots  was  then  forced  immediately.  The  re- 
mainder of  each  lot  was  kept  dormant  for  an  additional  month  before 
forcing.  Regardless  of  the  freezing  temperature,  the  roots  of  each  lot 
which  received  the  longer  rest  period  produced  a  two  to  three-fold 
greater  yield  and  made  a  more  rapid  growth  than  roots  which  had  not 
been  given  the  rest  period.  Severe  freezing  was  not  effective  in  short- 
ening the  rest  period  needed  for  maximum  growth. 


WINTER  FORCING  OF  RHUBARB 

By  C.  B.  SAYRE,  Formerly  Assistant  Chief  in  Olericulture 

Rhubarb  forcing  on  a  commercial  scale  is  a  profitable  enterprise 
in  Illinois  and  thruout  the  Northern  states.  In  some  localities  in  Illi- 
nois large  areas  are  devoted  to  the  growing  of  rhubarb  roots  exclus- 
ively for  forcing.  Many  commercial  gardeners  find  the  enterprise  a 
profitable  means  of  using  the  surplus  roots  which  are  available  when- 
ever a  rhubarb  plantation  is  renewed. 

Rhubarb  thrives  well  in  all  sections  of  Illinois.  Naturally  a  north- 
ern crop,  it  is  not  well  suited  to  southern  truck  farming.  Consequently 
to  force  it  is  by  far  the  best  method  of  producing  it  for  early  market 
in  the  North.  The  market  for  the  forced  product  can  be  considerably 
developed  in  this  state,  particularly  in  the  smaller  cities.1  The  fact 
that  forced  rhubarb,  when  properly  grown,  is  greatly  superior  to  that 
grown  out-of-doors  increases  the  security  of  the  commercial  enterprise. 
It  is  much  more  attractive  in  color  and  appearance  and  is  superior  in 
texture  and  flavor,  containing  less  acid.  It  is  deep  red  or  pink  in  color, 
and  when  cooked,  produces  a  beautiful  clear  pink  sauce.  The  stalks 
are  unusually  crisp,  tender,  and  free  from  strings  and  woody  fiber. 

In  addition  to  the  commercial  phase,  rhubarb  forcing  is  of  im- 
portance to  home  gardeners  and  farmers,  for  a  few  rhubarb  roots  in 
the  cellar  will  supply  the  home  table  with  this  delicious  product  dur- 
ing the  winter  months. 

COMMERCIAL  PRACTICES  IN  FORCING  RHUBARB 

Practices  in  forcing  rhubarb  vary  somewhat  with  different  pro- 
ducers, but  the  general  principles  are  similar.  When  the  rhubarb  roots 
are  grown  exclusively  for  forcing,  most  of  the  growers  prefer  to  use 
two-year-old  roots,  tho  a  few  use  one-year-old  roots.  When  rhubarb 
is  grown  principally  as  an  outdoor  crop  and  forcing  is  considered  only 
a  side  line  by  means  of  which  the  surplus  roots  are  profitably  utilized, 
the  ones  used  for  forcing  are  generally  five  or  six  years  old  or  more. 


'In  one  farmers'  retail  public  market  in  a  medium-sized  city  in  Illinois,  it  is 
not  unusual  to  sell  over  a  thousand  pounds  of  forced  rhubarb  in  a  single  market 
day.  The  only  competition  is  rhubarb  from  California  which  is  shipped  to  Illi- 
nois markets  principally  during  the  month  of  March.  In  1925  125  carloads  of 
California  rhubarb  were  shipped  to  Chicago.  However,  rhubarb  forced  in  Illinois 
can  successfully  compete  with  rhubarb  shipped  in  from  California  because  the 
forced  rhubarb  can  be  produced  earlier  in  the  winter,  is  of  superior  flavor,  and 
can  be  marketed  at  less  expense. 

55 


56 


BULLETIN  No.  298 


[October, 


In  any  case,  the  roots  are  dug  late  in  the  fall  and  left  outdoors 
until  they  have  become  thoroly  frozen,  after  which  they  are  ready  for 
forcing.  The  roots  should  not  be  dug  until  the  tops  have  died,  because 
a  translocation  of  food  materials  takes  place  and  the  more  food  ma- 
terial that  is  stored  in  the  roots  the  greater  will  be  the  yield  of  forced 
rhubarb. 


FIG.  1. — A  FIVE-YEAR-OLD  RHUBARB  ROOT 
Rhubarb  produces  large  fleshy  roots  in  which  an 
abundance  of  food  material  is  stored.  By  digging  these 
roots  after  growth  has  stopped  in  the  fall,  and  placing 
them  under  favorable  conditions  of  temperature  and 
moisture,  the  food  material  stored  in  them  is  utilized  to 
produce  forced  rhubarb.  The  above  roots  were  used  in 
the  experiment  to  determine  the  effect  of  different  tem- 
peratures on  the  yield  and  color  of  forced  rhubarb  de- 
scribed on  pages  59  to  61. 


When  the  roots  are  left  out  to  freeze,  they  should  be  covered  with 
a  little  soil  or  straw  to  check  excessive  evaporation.  The  usual  prac- 
tice is  to  bed  the  roots  on  the  surface  of  the  ground  in  the  forcing  shed, 
which  is  left  open  and  with  the  roof  off  until  after  the  roots  have  be- 
come frozen. 

Forcing  structures  differ  widely  in  size  and  type  of  construction. 
Fig.  2  shows  a  very  satisfactory  type  of  forcing  house.  This  house  is 
of  cheap  construction  and  is  dismantled  at  the  end  of  the  forcing  sea- 
son and  erected  in  a  new  location  the  next  year.  In  the  fall  the  frame- 
work is  erected  and  at  digging  time  the  roots  are  bedded  within  the 
area  to  be  covered  by  the  house.  After  the  roots  have  become  thoroly 


1927]  WINTER  FORCING  OF  RHUBARB  57 

frozen,  the  roof  and  sides  are  put  on  the  house  and  the  temperature 
raised  by  starting  the  stoves. 

Small  coal  stoves,  as  shown  in  Fig.  3,  are  placed  along  the  walls 
at  intervals  of  about  40  feet.  Each  stove  is  placed  just  inside  a  door 
of  the  house,  with  room  for  a  coal  box  and  for  firing.  To  make  a  more 
uniform  distribution  of  heat,  the  stove  pipes  extend  almost  horizontally 
toward  the  center  of  the  interval  between  stoves.  They  then  run  verti- 
cally thru  the  roof  and  appear  outside  in  pairs,  close  together,  altho 
their  respective  stoves  are  40  feet  apart.  The  arrangement  of  the  pipes 
is  shown  in  Fig.  2.  One  carload  of  coal  was  sufficient  to  maintain  a  sat- 
isfactory forcing  temperature  in  this  house  thruout  the  forcing  season. 


FIG.  2. — A  COMMERCIAL  RHUBARB  FORCING  HOUSE 

The  roots  were  bedded  in  the  fall  within  the  area  to  be  occupied.  After  they 
became  frozen,  the  house  was  erected  over  them  and  forcing  commenced.  This 
house  is  160  feet  long  and  48  feet  wide.  The  ridge  extending  lengthwise  thru  the 
center  is  6%  feet  high  and  the  eaves  5%  feet.  It  is  made  of  second-hand  lumber 
covered  with  tar  paper.  The  frame  is  strong  enough  to  sustain  the  weight  of  a 
layer  of  manure  a  foot  deep  over  the  roof.  Manure  is  likewise  banked  up  about 
3  feet  high  around  the  sides.  This  maintains  a  more  even  temperature  and  greatly 
reduces  the  expense  of  heating.  There  are  no  windows,  all  harvesting  being  done 
by  lantern  light. 

About  2  feet  above  the  beds  braces  are  nailed  to  the  posts  which 
support  the  roof.  By  laying  a  board  across  these  braces,  a  "bridge" 
is  provided  from  which  the  crop  is  harvested.  In  this  way  maximum 
cropping  area  is  secured  and  no  space  is  lost  in  aisles  or  alleys. 

At  the  end  of  the  forcing  season  this  house  is  dismantled  and  re- 
moved and  the  lumber  stacked  for  use  the  next  season.  The  exhausted 
roots  are  then  plowed  under,  which  does  away  with  the  labor  of  re- 
moving them. 

In  these  houses  no  flooring  is  used  and  the  roots  are  bedded  on 
the  surface  soil.  Dirt  is  then  filled  in  around  the  roots  to  the  level  of 
the  crowns.  By  having  no  roof  over  them  until  the  forcing  season  com- 
mences, the  roots  receive  the  full  benefit  of  any  rain  or  snowfall.  This 
provides  sufficient  moisture  to  start  growth  when  the  temperature  is 
raised. 

Another  type  of  forcing  structure  used  by  market  gardeners  is  a 
celery  storage  house.  The  celery  is  out  of  the  way  before  the  rhubarb 


58 


BULLETIN  No.  298 


[October, 


forcing  season  commences  and  the  dirt  floor,  well-insulated  walls,  and 
subdued  light  furnish  favorable  conditions  for  both  crops.  If  a  celery 
storage  house  is  used  for  rhubarb,  the  rhubarb  roots  are  dug  before 
freezing  weather  and  left  in  an  exposed  place  to  freeze  until  the  celery 
is  removed  from  the  house.  The  rhubarb  is  then  bedded  on  the  dirt 
floor,  as  previously  described. 

Regardless  of  the  type  of  forc- 
ing house,  the  aim  in  commercial 
forcing  is  to  produce  large  yields 
of  deep  red  or  pink-colored  stalks. 
The  deeper  the  red  color,  the  more 
desirable  is  the  product  from  a 
market  standpoint.  Forced  rhu- 
barb is  harvested  when  the  stalks 
are  18  inches  or  more  long.  The 
stalks  are  graded  into  three 
lengths  (18,  20,  and  22  inches) 
and  are  generally  packed  in  boxes 
holding  20  pounds.  The  boxes  are 
lined  with  paper  and  packed  at- 
tractively, in  keeping  with  the 
high  quality  of  the  product.  The 
stalks  are  sometimes  tied  in 
bunches  of  three  or  four  stalks, 
but  usually  are  not  bunched.  They 
are  sold  by  the  pound. 

Illinois  markets  prefer  forced 
rhubarb  grown  in  the  dark  or  sub- 
dued light.  In  this  case  the  leaf 
blades  are  very  small  and  unde- 
veloped, as  shown  in  Fig.  5  and  in 
the  colored  plate. 


FIG.  3. — SMALL  COAL  STOVES  ARE  USED 
FOR  HEATING  THE  FORCING  HOUSES 
Eight  of  these  stoves  furnished  suf- 
ficient heat  for  a  house  160  feet  long 
and  48  feet  wide. 


RHUBARB  FORCED  IN  CELLAR  FOR  HOME  USE 

For  home  use  rhubarb  may  be  forced  in  a  cellar,  as  shown  in  Fig. 
5.  After  the  roots  have  been  frozen,  they  are  bedded  close  together  on 
the  cellar  floor  and  dirt  is  filled  in  around  the  roots.  Ashes  or  sand 
may  be  used  in  place  of  soil,  as  the  function  of  the  soil  is  merely  to 
hold  moisture  for  the  roots.  Four  or  five  large  roots  will  supply  all  the 
rhubarb  an  average  family  will  use  during  the  winter.  The  roots 
should  be  placed  in  a  darkened  part  of  the  cellar  and  protected  from 
direct  light.  A  subdued  light  does  no  harm,  but  direct  light  will  result 
in  large  leaves  and  crooked  stems,  because  the  stalks  will  bend  toward 
the  light.  By  placing  brown  paper  over  the  windows,  a  satisfactory 
subdued  light  may  be  secured  that  will  not  affect  the  growth. 


1927]  WINTER  FORCING  OF  RHUBARB  59 

EXPERIMENTS  TO  FIND  BEST  METHODS 
OF  FORCING  RHUBARB 

The  variation  in  commercial  practices  and  in  opinions  in  regard 
to  the  best  methods  of  forcing  rhubarb,  together  with  the  general  lack 
of  scientific  data  on  the  subject,  prompted  this  series  of  experiments. 
Altho  rhubarb  forcing  has  been  practiced  for  over  half  a  century,  not 
much  has  been  written  on  the  subject.  A  few  popular  articles  have 
appeared  from  time  to  time,  but  most  of  the  literature  has  been  of  a 
general  nature  and  principally  a  description  of  commercial  practices. 

It  therefore  was  the  object  of  these  investigations  to  obtain  infor- 
mation in  regard  to  the  following  points:  (1)  the  effect  of  different 
temperatures  on  the  yield  and  color  of  forced  rhubarb,  and  the  most 
favorable  forcing  temperature;  (2)  the  effect  of  watering  the  roots, 
as  a  factor  determining  the  yield  and  color  of  forced  rhubarb;  (3)  the 
best  age  of  roots  for  forcing;  (4)  the  effect  of  different  intensities  of 
freezing  and  different  lengths  of  freezing  periods  in  breaking  the  rest 
period  and  increasing  yields;  and  (5)  the  length  of  rest  period  needed 
to  produce  maximum  yields. 

EFFECT  OF  DIFFERENT  TEMPERATURES  ON  YIELD 
AND  COLOR  OF  FORCED  RHUBARB 

In  the  literature  on  rhubarb  forcing  there  is  a  marked  range  in  the 
recommendations  as  to  the  best  forcing  temperature.  In  fact,  the  prin- 
cipal treatise  on  rhubarb  forcing  says,  "The  temperature  may  vary 
from  45°  to  80°  or  90°  according  to  circumstances."1  Practices  vary 
with  different  growers,  some  preferring  a  temperature  of  50°,  some  60°, 
and  some  using  75°  F.  As  might  be  expected,  yields  and  quality  of 
product  vary  under  these  different  temperatures. 

To  determine  the  effect  of  different  temperatures  on  the  yield 
and  color  of  forced  rhubarb  some  five-year-old  roots  that  were  of  the 
same  type  and  had  been  grown  under  identical  conditions  were  selected 
for  experiment  (Fig.  1).  From  a  large  number  of  roots  clumps  were 
chosen  of  equal  size  and  weighing  21  pounds  each.  These  were  bedded 
in  forcing  boxes  and  placed  in  special  thermostatically  controlled  tem- 
perature chambers.2  The  chambers  are  electrically  connected  with 
heaters  and  refrigerating  coils,  so  that  a  constant  temperature  may  be 
maintained  at  all  times  regardless  of  exterior  changes  of  temperature. 

A  selected  rhubarb  clump  was  placed  in  each  of  the  following  tem- 
peratures: 50°,  59°,  68°,  77°,  and  86°  F.  These  temperatures  were 
maintained  constantly  thruout  the  forcing  period.  All  the  roots  were 

'The  New  Rhubarb  Culture.  J.  E.  Morse.  1901. 

"These  chambers  are  in  the  plant  physiology  laboratory  of  the  Department 
of  Botany,  the  author  being  permitted  to  use  them  thru  the  courtesy  of  Dr.  C.  F. 
Hottes.  The  experiments  herein  reported  were  conducted  during  the  years  1922  to 
1925. 


60  BULLETIN  No.  298  [October, 

bedded  in  the  same  way  on  March  30,  and  care  was  taken  to  see  that 
other  growing  conditions  were  the  same  in  each  lot.  Temperature,  con- 
sequently, was  the  only  factor  in  which  the  different  lots  varied,  so 
that  differences  in  yield  and  color  of  the  forced  rhubarb  may  be  at- 
tributed to  differences  in  the  forcing  temperature. 

As  each  lot  was  harvested,  the  number  of  marketable  stalks  and 
their  total  weight  were  recorded.  A  typical  specimen  of  each  lot  was 
selected  and  compared  with  the  color  charts  in  Ridgway's  "Color 
Standards  and  Nomenclature,"  and  an  accurate  color  record  thus  ob- 
tained. Since  the  color  is  always  darker  near  the  base  of  the  stalks 
and  lighter  at  the  top,  the  color  at  three  places  on  each  stalk  was  re- 
corded; namely,  2  inches  from  the  base,  at  the  center,  and  2  inches 
from  the  top  of  the  stalk.  In  all  cases  it  was  the  color  on  the  exterior, 
or  convex,  side  of  the  stalk  that  was  recorded. 

Different  Temperatures  Produce  Striking 
Differences  in  Yield 

Some  striking  differences  will  be  noted  in  the  yields  of  these  lots, 
that  were  due  to  differences  in  forcing  temperatures  (Table  1). 

Of  the  five  temperatures  used,  59°  F.  produced  the  largest  yields. 
An  earlier  crop  was  produced  at  68°  F.  but  the  yield  was  consider- 
ably less  than  at  59°  F. 

Temperatures  of  77°  and  86°  F.  proved  excessive;  weak,  spind- 
ling growth  resulting  which  quickly  succumbed  to  soft  rot.  Unques- 
tionably 77°  F.  is  too  high  a  temperature  to  maintain  thruout  the  forc- 
ing period.  Observations  in  commercial  forcing  houses  show  that  this 
temperature  may  be  tolerated  for  brief  intervals  without  serious  in- 
jury to  the  crop,  but  it  is  too  high  a  temperature  to  maintain  contin- 
uously. 

Fifty  degrees  evidently  is  below  the  best  forcing  temperature.  Al- 
tho  a  marketable  crop  of  high  quality  was  grown  at  this  temperature, 
the  growth  was  very  slow  and  the  yield  was  23  percent  less  than  at 
59°  F. 

A  comparison  of  the  number  of  days  it  took  the  three  lots  to  pro- 
duce their  crops  of  marketable  stalks,  and  the  number  of  days  until 
the  first  picking,  is  shown  by  the  following  figures: 

Lot  1 — 50°  F.  87  days  to  produce  crop,  33  days  for  first  picking 
Lot  2 — 59°  F.  83  days  to  produce  crop,  25  days  for  first  picking 
Lot  3 — 68°  F.  62  days  to  produce  crop,  18  days  for  first  picking 

As  might  be  expected,  the  higher  the  forcing  temperature,  the 
quicker  the  crop  was  produced.  Each  9  degrees  increase  in  tempera- 
ture brought  the  first  picking  7  days  earlier.  The  extremely  rapid 
growth,  however,  did  not  yield  the  largest  crop.  As  stated  above,  the 
most  favorable  temperature  for  large  yields  proved  to  be  59°  F.,  altho 
68°  F.  produced  a  much  earlier  crop.  On  the  basis  of  both  earliness 


WINTER  FORCING  OF  RHUBARB 


61 


and  yield,  both  of  these  higher  temperatures  proved  better  than  50°  F., 
which  was  used  for  Lot  1. 

Evidently  the  temperature  used  in  forcing  rhubarb  is  an  important 
factor  affecting  both  the  total  yield  and  the  rate  of  growth. 


TABLE  1. — YIELDS  OF  RHUBARB  AT  DIFFERENT  FORCING  TEMPERATURES 

(Five-year-old  roots.    Frozen  in  field  all  winter,  dug 

March  23,  bedded  March  30) 


Date 

Lotl,  forced  at  50°F.; 
marketable  stalks 
produced 

Lot  2,  forced  at  59°F.; 
marketable  stalks 
produced 

Lot  3,  forced  at  68°F.; 
marketable  stalks 
produced 

Number 

Weight 

Number 

Weight 

Number 

Weight 

Apr.  16  

grams 

247 
171 
101 
267 
450 
223 
368 
354 
176 
499 
349 
154 

"9 
6 
10 
5 
8 
3 
7 
10 
9 
13 
8 
7 
13 
7 

grams 

399 
355 
485 
169 
326 
139 
359 
389 
391 
383 
272 
251 
332 
158 

4 
11 
8 
8 
12 
7 
14 
5 
7 
11 
12 
7 
6 
3 

grams 
221 
487 
315 
301 
451 
271 
438 
148 
262 
360 
331 
200 
165 
80 

Apr.  18. 
Apr.  20. 
Apr.  23. 
Apr.  27. 
Apr.  30. 
May  2  

"e 

4 
3 
7 
12 
4 
9 
8 
5 
11 
11 
5 

May  4  

May  7  

May  11.      . 
May  16. 
May  21  .      . 
May  26  .      . 
May  31  .      . 
June  5  .  . 

June  14  . 
June  21  .      . 
June  25  . 

Total  

85 

3  359  or 
7  Ibs.  6  oz. 

115 

4  408  or 
9  Ibs.  lloz. 

115 

4  030  or 
8  Ibs.  14  oz. 

Lots  4  and  5,  forced  at  77°  and  86°  F.  respectively,  did  not  produce  any  market- 
able rhubarb.  The  growth  was  weak  and  spindling  and  in  two  weeks  collapsed  from 
soft  rot.  Apparently  these  temperatures  are  too  high  for  healthy  growth.  Shoots 
appeared  first  in  Lot  3,  followed  in  order  by  Lots  4,  2,  5,  and  1. 


Lower  Temperatures  Result  in  Deeper  Color 

Color  is  an  important  factor  in  the  market  value  of  forced  rhubarb. 
The  more  red  or  dark  pink  in  the  stalks  the  more  attractive  their  ap- 
pearance and  the  higher  the  price  they  will  command.  The  color  is  re- 
tained in 'the  cooked  rhubarb  and  adds  greatly  to  its  attractiveness 
when  served. 

In  Table  2  are  given  accurate  color  records  of  the  yields  produced 
by  Lots  1,  2,  and  3.  Three  points  should  be  noted,  namely: 

1.  The  lower  the  forcing  temperature,  the  darker  red  is  the  rhu- 
barb produced,  and  this  relative  difference  persists  thruout  the  forcing 
period. 


62 


BULLETIN  No.  298 


[October, 


2.  At  50°  F.  the  deep  red  color  extends  the  full  length  of  the  stalk. 
This  valuable  characteristic  continued  thruout  the  forcing  period.  The 


TABLE  2. — COLOR  RECORD  OF  RHUBARB  FORCED  AT  DIFFERENT  TEMPERATURES 


Date 


2  inches  from  base 
of  stalk 


Center  of  stalk 


2  inches  from  top 


Lot  1.     Forced  at  50°F. 


May  2 Carmine  Carmine  Carmine 

May  4 Scarlet  red  Spectrum  red  Spectrum  red 

May  7 Carmine  Carmine  Nopal  red 

May  11 Carmine  Carmine  Carmine 

May  16 Carmine  Carmine  Carmine 

May  21 Ox  blood  red  Ox  blood  red  Carmine 

May  26 Nopal  red  Nopal  red  Nopal  red 

May  31 Scarlet  red  Scarlet  red  Rose  doree 

June  5 Scarlet  red  Spectrum  red  Rose  doree 

June  14 Scarlet  red  Scarlet  red  Rose  doree 

June  21 Spectrum  red  Rose  doree  Geranium  pink 

June  25 Scarlet  red Rose  doree Rose  doree 

Lot  2.     Forced  at  59°  F. 

Apr.  23 Scarlet  red  Spectrum  red  Rose  doree 

Apr.  27 Scarlet  red  Rose  doree  Geranium  pink 

Apr.  30 Scarlet  red  Rose  doree  Shrimp  pink 

May  2 Rose  doree  Geranium  pink  Martius  yellow 

May  4 Scarlet  red  Rose  doree  Shrimp  pink 

May  7 Scarlet  red  Begonia  rose  La  France  pink 

May  11 Spectrum  red  Scarlet  red  Rose  doree 

May  16 Scarlet  red  Rose  doree  Rose  doree 

May  21 Spectrum  red  Rose  doree  Geranium  pink 

May  26 Spectrum  red  Rose  doree  Shrimp  pink 

May  31      . .  Rose  doree  Geranium  pink  Shrimp  pink 

June  5 Begonia  rose  Begonia  rose  Geranium  pink 

June  14 Rose  doree  Rose  doree  Orient  pink 

June  21 Rose  doree Geranium  pink Shrimp  pink 

Lot  3.     Forced  at  68°  F. 

Apr.  20 Scarlet  Rose  doree  Shrimp  pink 

Apr.  23 Scarlet  Rose  doree  Shrimp  pink 

Apr.  27 Rose  doree  Geranium  pink  Shrimp  pink 

Apr.  30 Rose  doree  Geranium  pink  Shrimp  pink 

May  2 Geranium  pink  Shrimp  pink  Baryta  yellow 

May  4 Rose  doree  La  France  pink  Shrimp  pink 

May  7 Geranium  pink  Eosine  pink  Baryta  yellow 

May  11 Begonia  rose  La  France  pink  Shrimp  pink 

May  16 Geranium  pink  Shrimp  pink  Sulfur  yellow 

May  21 Rose  doree  La  France  pink  Sulfur  yellow 

May  26 Eosine  pink  Shrimp  pink  Sulfur  yellow 

May  31 Geranium  pink Shrimp  pink Sulfur  yellow 

NOTE. — For  the  benefit  of  those  who  do  not  have  available  plates  of  standard 
colors  and  nomenclature,  it  should  be  said  that  ox  blood  red,  nopal  red,  carmine, 
scarlet  red,  and  spectrum  red  are  all  deep  shades  of  red,  and  that  rose  doree,  begonia  _ 
rose,  geranium  pink,  La  France  pink,  and  shrimp  pink  are  shades  of  pink  ranging 
from  dark  to  light  in  the  order  named.  Stalks  typical  of  the  colors  produced  at  the 
different  temperatures  are  shown  in  the  accompanying  colored  plate. 


1927]  WINTER  FORCING  OF  RHUBARB  63 

red  color  also  extended  into  the  large  veins  of  the  leaves,  while  at 
higher  temperatures  only  yellow  and  light  green  appeared  in  the  leaves. 

3.  The  red  color  becomes  less  intense  as  the  harvesting  season 
continues.  This  should  be  noted  also  in  the  records  of  Lots  6,  7,  8,  and 
9,  Tables  4  and  6. 

Since  the  color  of  the  rhubarb  forced  at  59°  F.  was  entirely  sat- 
isfactory, and  the  yield  was  30  percent  greater  (by  weight)  and  the 
growth  quicker  than  at  50°  F.,  it  would  seem  best  for  growers  to  use 
the  higher  temperature  under  most  circumstances.  At  68°  F.  the  tem- 
perature was  evidently  above  the  optimum,  for  altho  the  growth  was 
quicker  the  yield  was  less  and  the  color  was  considerably  inferior. 

YIELD  AND  COLOR  AS  AFFECTED  BY  WATERING 
Many  commercial  growers  produce  the  entire  crop  of  forced  rhu- 
barb without  watering  the  roots  at  all  during  the  forcing  period.  This 
method  is  commonly  practiced  where  the  roots  are  bedded  in  the  open 
in  the  fall  or  where  the  roof  is  left  off  the  forcing  house  until  after  the 
roots  have  frozen.  In  this  way  the  roots  and  the  soil  packed  around 
them  receive  the  full  benefit  of  any  rain  or  snowfall  until  the  roof 
is  put  on  and  the  forcing  period  commences.  Some  growers  claim  that 
this  furnishes  sufficient  moisture  for  good  yields  of  forced  rhubarb, 
making  additional  expense  for  watering  unnecessary.  Others  claim  that 
watering  the  roots  during  the  forcing  period  will  "wash  out"  the  color 
and  therefore  is  undesirable  since  the  darker  red  stalks  bring  the 
higher  price. 

Since  the  marketable  product  of  forced  rhubarb  is  over  95  percent 
water,  and  since  this  forced  growth  results  when  water  is  taken  up  by 
the  roots  and  the  food  materials  stored  in  the  roots  thus  utilized,  it 
seemed  to  the  author  that  timely  watering  during  the  forcing  period 
would  increase  the  yields.  Accordingly  the  following  test  was  made  in 
order  to  determine  if  watering  the  roots  during  the  forcing  period 
would  increase  the  yields  and  whether  it  would  result  in  paler,  less 
desirable  stalks. 

From  a  large  number  of  five-year-old  rhubarb  clumps,  twelve 
were  selected  that  were  of  equal  size  and  weight.  These  were  all  of  the 
same  variety  and  had  been  grown  under  similar  conditions  in  the  gar- 
den. These  large,  uniform  clumps  were  divided  into  two  lots  of  six 
each  and  on  March  30  were  bedded  in  moist  soil  in  adjacent  sections 
of  the  forcing  cellar.  These  lots  were  grown  and  forced  under  identical 
conditions  except  that  one  lot,  No.  6,  was  watered  at  timely  intervals 
during  the  forcing  period,  while  the  other  lot,  No.  7,  was  not  watered. 
No  schedule  was  followed  in  watering  Lot  6,  the  water  being  applied 
whenever  the  soil  seemed  dry  and  it  was  thought  watering  would  be 
beneficial.  All  of  these  roots  had  been  frozen  in  the  field,  where  they 


PLATE  I 

Typical  color  of  stalks  at  different  forcing  temperatures. 

A  and  C— Forced  at  59°  F. 
B  and  D— Forced  at  68°  F. 
E  —Forced  at  50°  F. 

The  lower  the  forcing  temperature  the  slower  the  growth  and  the 
darker  the  color.  At  low  temperatures  the  red  or  dark  pink  color  ex- 
tends the  full  length  of  the  stalks  and  into  the  veins  of  the  leaves. 
A  deep  pink  color  is  preferred  in  the  markets.  At  59°  F.  the  largest 
yield  was  produced  and  the  stalks  were  of  good  marketable  color. 


64 


Rhubarb  Forcing  Experiments 


PLATE  I 


Illinois  Agricultural  Experiment  Station 


1927] 


WINTER  FORCING  OF  RHUBARB 


65 


received  full  benefit  of  rain  and  snowfall.  They  were  dug  before 
growth  started  in  the  spring  and  were  immediately  bedded  in  moist 
soil  in  the  forcing  cellar. 

Watering  Increases  Yield 

The  stalks  were  harvested  as  they  reached  marketable  size  and  a 
record  was  kept  of  the  number  of  marketable  stalks  and  total  weight 
of  each  harvest  (Table  3). 

The  result  of  watering  Lot  6  was  that  the  yield  was  increased 
threefold.  At  15  cents  a  pound,  which  is  an  average  wholesale  price, 


TABLE  3. — EFFECT  OF  WATERING  ON  YIELD  OF  FORCED  RHUBARB 

(Five-year-old  roots,  6  in  each  lot.   Frozen  in  field  all  winter,  dug  March  23,  bedded 

March  30.     Forced  at  55°  F.) 


Date 

Lot  6,  watered  during  forcing; 
marketable  stalks  produced 

Lot  7,  not  watered;  marketable 
stalks  produced 

Number 

Weight 

Number 

Weight 

Apr  26 

16 
29 
34 
34 
39 
37 
44 
54 
54 

Ibs.  oz. 
2—  6 
4—  2 
5—  1 
4—  9 
4—  9 
3—13 
3—12 
5—  4 
3—  4 

is 

17 
13 
15 
22 
18 
24 

Ibs.    oz. 

Apr  28 

May  1  

2 
1—15 
1—  9 
1—  7 
1—14 
1—  8 
1—10 

May  4  

May  8  

May  12  

May  16  

May  26  

June  6  

Total  

341 

36—12 

122 

11—15 

Average  yield  per  root  

6—  2 

2 

it  may  be  said  that  timely  watering  increased  the  returns  62  cents  a 
root.  All  the  other  lots  included  in  these  experiments  were  watered 
during  the  forcing  period,  and  this  treatment  doubtless  was  a  factor  in 
the  large  average  yields  produced  in  most  of  the  lots. 

Color  Not  "Washed  Out"  by  Watering 

The  benefit  of  increased  yields  resulting  from  watering  would  be 
greatly  offset  if  watering  had  a  tendency  to  "wash  out"  the  color  of 
the  marketable  product.  Consequently  an  accurate  record  of  the  color 
of  the  stalks  produced  at  each  harvest  period  is  of  significant  com- 
mercial value.  This  is  given  in  Table  4. 

At  the  end  of  the.  harvesting  season  the  stalks  from  Lot  7  (un- 
watered)  were  of  a  darker  red  than  the  stalks  from  Lot  6.  However, 
it  is  necessary  to  consider  another  factor  in  this  connection,  namely, 
that  as  the  roots  become  exhausted  the  stalks  produced  are  of  a  less 
intense  red.  Since  the  roots  that  had  been  watered  produced  over  three 


66 


BULLETIN  No.  298 


[October, 


times  as  much  marketable  rhubarb  as  the  unwatered  roots,  it  is  to  be 
expected  that  at  the  end  of  the  season  the  roots  in  Lot  6  would  be 
more  exhausted,  and  therefore  would  produce  lighter  colored  stalks 
than  Lot  7.  This  is  substantiated  by  the  fact  that  when  Lot  6  had 
produced  2  pounds  of  marketable  rhubarb  per  root  (equal  to  the 
total  yield  of  Lot  7) ,  the  color  of  the  stalks  was  practically  the  same 
dark  pink  as  were  those  in  Lot  7  at  the  end  of  the  season.  It  would 
therefore  seem  that  watering  the  roots  during  the  forcing  period  did 
not  "wash  out"  the  color  of  the  stalks.  It  should  be  noted  that  the 
watered  roots  produced  a  heavy  crop  of  good  marketable  color  and 
grade  thruout. 

A  word  of  caution  in  regard  to  over-watering  might  be  mentioned. 
The  same  principles  would  apply  in  watering  forced  rhubarb  as  in 


TABLE  4. — COLOR  RECORD  OP  RHUBARB  WHEN  WATERED  AND  WHEN 

NOT  WATERED 


Date 

2  inches  from  base 

Center  of  stalk 

2  inches  from  top 

Lot  6.     Watered  during  forcing 


Apr.  26  

Rose  doree 

Rose  doree 

La  France  pink 

Apr.  28  

Scarlet  red 

Rose  doree 

La  France  pink 

May  1  

Scarlet  red 

Scarlet  red 

La  France  pink 

May  4  

Scarlet  red 

Rose  doree 

Shrimp  pink 

May  8  

Scarlet  red 

Geranium  pink 

Sulfur  yellow 

May  12  

Spectrum  red 

Rose  doree 

Shrimp  pink 

May  16  

Rose  doree 

Rose  doree 

Shrimp  pink 

May  26  

Rose  doree 

Geranium  pink 

Sulfur  yellow 

June  6  

Rose  doree 

Geranium  pink 

Sulfur  yellow 

Lot  7.     Not  watered 


May  1  

Scarlet  red 

Rose  doree 

Shrimp  pink 

May  4  

Nopal  red 

Scarlet  red 

La  France  pink 

May  8  

Nopal  red 

Scarlet  red 

Shrimp  pink 

May  12  

Ox  blood  red 

Nopal  red 

Shrimp  pink 

May  16  

Nopal  red 

Nopal  red 

La  France  pink 

May  26  

Ox  blood  red 

Carmine 

Shrimp  pink 

June  6  

Spectrum  red 

Geranium  pink 

Shrimp  pink 

watering  any  other  forced  crop;  namely,  to  water  thoroly  at  each  wa- 
tering, avoiding  frequent  light  sprinklings  which  would  tend  to  main- 
tain a  moist  surface  and  favor  the  development  of  disease  organisms. 

BEST  AGE  OF  ROOTS  FOR  FORCING 

The  best  age  of  rhubarb  roots  for  forcing  cannot  be  determined 
merely  by  comparing  the  yields  produced.  Various  other  factors  must 
be  considered,  such  as  the  cost  of  growing  the  roots  of  each  age,  the 
added  returns  from  outdoor  harvesting,  costs  of  digging  roots,  and 
earliness  and  quality  of  stalks  from  roots  of  each  age,  and  whether 


19S7] 


WINTER  FORCING  OF  RHUBARB 


67 


the  roots  are  grown  primarily  for  forcing  or  whether  the  forcing  is  a 
side  line  incidental  to  field  growing. 

In  order  to  obtain  some  information  in  regard  to  the  yield  and 
quality  of  rhubarb  from  roots  of  different  ages,  the  following  investiga- 
tions extending  over  three  seasons  were  undertaken.  The  results  indi- 
cate the  yields  that  may  be  expected  from  well-grown  roots  of  each  age. 

Five-Year  Roots  Prove  Superior  to  One- Year  Roots 

During  the  first  season  only  one-year-old  roots  and  five-year-old 
roots  were  included  in  the  experiment  (Table  5).  These  were  dug  on 
March  23,  and  twenty-five  uniform  clumps  of  each  age  selected.  They 
had  all  been  frozen  in  the  field  during  the  winter  and  were  dug  before 
growth  started  in  the  spring.  The  next  day  both  lots  were  bedded  in 


TABLE  5. — YIELDS  OF  RHUBARB  FROM  ROOTS  OF  DIFFERENT  AGES 

(Twenty-five  roots  in  each  lot.    Frozen  in  field  all  winter,  dug  March  23,  bedded 

March  24.     Forced  at  55°  F.) 


Date 

Lot  8,  one-year-old  roots; 
marketable  stalks  produced 

Lot  9,  fiye-year-old  roots; 
marketable  stalks  produced 

Number 

Weight 

Number 

Weight 

Apr.  17  

99 

48 
114 
40 
45 
47 
82 
65 
39 
31 
19 

Ibs.    oz. 
18—  2 
9—  2 
16—  4 
4—15 
5—14 
5—  7 
8—  6 
5—  4 
2—  9 
2—  4 
1—  4 

76 
85 
172 
63 
55 
143 
90 
76 
75 
89 
117 
37 

Ibs.    oz. 
11—  2 
12—12 
25—  2 
9 
8—  6 
17—12 
11 
7—10 
6—14 
7—  5 
9 
2—  9 

Apr.  20  

Apr.  23  

Apr.  26  

Apr.  28  

May  1  

May  4  

May  8  

May  12  

May  16  

May  26  

June  6  

Total  

629 

79—  7 

1,078 

128—  8 

Average  yield  per  root  

3.  18  Ibs. 

5.  14  Ibs. 

adjacent  sections  of  the  forcing  cellar  and  placed  under  identical  forc- 
ing conditions  at  a  temperature  of  approximately  55°  F.  A  typical 
root  of  each  of  these  lots  is  shown  in  Fig.  4;  Fig.  5  shows  the  two  lots 
growing  in  the  forcing  cellar. 

On  April  17  the  first  stalks  were  harvested  from  these  lots.  The 
last  marketable  stalks  from  the  one-year  roots  were  harvested  on 
May  26;  the  five-year  roots  produced  one  more  pulling  on  June  6.  The 
total  yields  of  the  two  lots  are  shown  in  Table  5,  and  the  color  record 
of  the  stalks  in  Table  6.  The  stalks  produced  by  the  one-year  roots 
were  a  much  lighter  pink  than  those  produced  by  the  five-year  roots. 


68 


BULLETIN  No.  298 


[October, 


The  five-year-old  roots  produced  an  average  of  2  pounds  of  mar- 
ketable rhubarb  more  per  root  than  did  the  one-year-old  roots.  Like- 
wise the  color,  and  consequently  the  value,  of  the  product  from  the 


FIG.  4. — TYPICAL  ROOTS  FROM  LOTS  8  AND  9 

At  the  left  is  a  one-year-old  root  from  Lot  8,  at  the  right  a  five- 
year-old  root  from  Lot  9. 


Fia.  5. — TYPICAL  GROWTH  FROM  LOTS  8  AND  9 

The   five-year-old  roots   (Lot  9)    at  the  right   produced  heavier, 
darker-colored  stalks  than  the  one-year-old  roots  (Lot  8)  at  the  left. 


five-year-old  roots  was  considerably  superior  to  that  produced  by  the 
one-year-old  roots.  Taking  color  and  yield  into  consideration,  the  five- 
year-old  roots  were  better  than  the  one-year-old  roots  for  forcing. 
From  the  standpoint  of  expense  also,  the  older  roots  are  to  be  pre- 


1927]  WINTER  FORCING  OF  RHUBARB 

TABLE  6. — COLOR  RECORD  OF  RHUBARB  GROWN  FROM  ROOTS  OF  Two 

DIFFERENT  AGES 
(Roots  forced  at  55°  F.) 


Date 

2  inches  from  base 
of  stalk 

Center  of  stalk 

2  inches  from 
top  of  stalk 

Lot  8.     Grown  from  one-year-old  roots 


Apr.  20.. 

Geranium  pink 

Coral  pink 

Shrimp  pink 

Apr.  23  

Begonia  rose 

Geranium  pink 

Shrimp  pink 

Apr.  26  

Eosine  pink 

La  France  pink 

Orient  pink 

Apr.  28  

Eosine  pink 

La  France  pink 

Pale  salmon  pink 

May  1  

Eosine  pink 

La  France  pink 

Sulfur  yellow 

May  4  

La  France  pink 

Shrimp  pink 

Massicot  yellow 

May  8  

Shrimp  pink 

Shrimp  pink 

Sulfur  yellow 

May  12  

Shrimp  pink 

Pale  green-yellow 

Pale  green-yellow 

May  16  

Shrimp  pink 

Sulfur  yellow 

Sulfur  yellow 

May  26  

Shrimp  pink 

Sulfur  yellow 

Sulfur  yellow 

Lot  9.     Grown  fr 

om  five-year-old  root 

8 

Apr  20.. 

Scarlet 

Eosine  pink 

Shrimp  pink 

Apr.  23  

Scarlet  red 

Begonia  rose 

Eosine  pink 

Apr.  26  

Rose  doree 

Rose  doree 

La  France  pink 

Apr.  28  

Rose  doree 

Rose  doree 

Shrimp  pink 

May  1  

Geranium  pink 

Geranium  pink 

Shrimp  pink 

May  4  

Geranium  pink 

La  France  pink 

Martius  yellow 

May  8  

Rose  doree 

Eosine  pink 

Sulfur  yellow 

May  12  

Rose  doree 

Geranium  pink 

Shrimp  pink 

May  16  

Geranium  pink 

Geranium  pink 

Shrimp  pink 

May  26  

Geranium  pink 

La  France  pink 

Sulfur  yellow 

June  6  

Rose  doree 

Rose  doree 

Sulfur  yellow 

ferred.  From  the  second  year  on,  a  rhubarb  plantation  should  produce 
sufficient  income  from  the  field  crops  to  pay  for  the  roots,  but  no  in- 
come will  have  been  obtained  from  one-year  old  roots. 

Second  Year's  Test  Shows  Effect  of  Rest  Period 

During  the  second  year  of  this  experiment,  one-,  two-,  and  six- 
year-old  roots  were  forced  (Table  7).  The  clumps  were  all  dug  on 
November  21.  Six  lots  of  roots  were  selected,  each  consisting  of  ten 
roots.  All  roots  were  as  uniform  as  it  was  possible  to  select.  They 
were  all  of  the  same  variety  and  strain  and  had  been  grown  under 
similar  conditions  in  the  field. 

Lots  10,  11,  and  12,  consisting  of  one-,  two-,  and  six-year  roots 
respectively,  were  kept  in  a  cold  storage  chamber  at  20°  F.  for  two 
weeks,  being  put  into  the  freezing  temperature  on  November  23  and 
removed  December  7.  They  were  bedded  in  adjacent  sections  of  the 
forcing  cellar  on  December  10  and  grown  under  identical  forcing  con- 
ditions, the  forcing  temperature  being  approximately  58°  F. 

The  other  three  lots,  all  six-year  roots,  were  treated  somewhat 
differently.  Lot  13  was  frozen  for  one  week  only,  having  been  re- 


70 


BULLETIN  No.  298 


[October, 


moved  from  the  refrigerating  room  on  November  30.  Lot  14  was  not 
frozen  at  all  at  this  time.  Both  Lots  13  and  14  were  bedded  with  Lots 
10,  11,  and  12,  on  December  10. 

On  January  10,  one  month  later,  Lot  14  was  removed  from  the 
forcing  cellar  and  taken  outside  for  ten  days  to  freeze.  There  had 
been  no  sign  of  growth  on  any  of  the  roots  of  this  lot,  while  on  the 
other  four  lots  in  the  forcing  house  vigorous  growth  had  commenced 
and  shoots  10  inches  high  were  already  produced  on  each.  Lot  14  was 
again  bedded  on  January  21,  in  the  forcing  cellar,  and  Lot  15,  which 
had  been  left  outdoors  to  freeze  for  eight  weeks,  was  also  bedded  at 
this  time. 

No  color  records  were  kept  of  this  group  of  experiments,  but  the 
yields  are  recorded  in  Table  7. 


TABLE  7. — YIELDS  OP  FORCED  RHUBARB  FROM  ROOTS  OF  DIFFERENT  AGES 
(Ten  roots  in  each  lot,  bedded  December  10.   Forced  at  approximately  58°  F.) 


Lot 

Age  of 
roots 

Length  of 
freezing 

First 
picking 

Last 
picking 

Total 
yield 

Yield 
per  root 

10 

1  year  

2  weeks  at  20°  .  .  . 

Jan.  21 

Feb.  26 

Ibs.    oz. 
22—  4 

Ibs. 
2.22 

11 

2  years  

2  weeks  at  20°  .  .  . 

Jan.  25 

Feb.  26 

14—13 

1.48 

12 

6  years  

2  weeks  at  20°.  .  . 

Jan.  28 

Feb.  21 

18—15 

1.90 

13 

6  years  

1  week  at  20°  

Jan.  21 

Mar.  10 

23—  8 

2.35 

14 

6  years  

Tried  unfrozen, 

15 

6  years  

later  frozen1  .  .  . 
8  weeks  outdoors2 

Mar.  10 
Feb.  26 

Apr.  8 
Apr.  8 

10—  2 
54—  9 

1.16 
5.45 

*No  growth  resulted  after  one  month  in  forcing  temperature.  Removed  on  Janu- 
ary 10  for  freezing  and  rebedded  on  January  21. 

2Bedded  January  21. 

NOTE. — The  low  yield  of  all  lots  except  No.  15  apparently  was  due  to  the  roots 
not  having  sufficient  rest  period  before  being  forced.  Consequently,  these  results 
should  be  disregarded  as  a  test  of  the  best  age  of  roots  for  forcing. 


The  disappointingly  low  yields  from  Lots  10,  11,  and  12,  having 
identical  treatment,  and  from  Lot  13,  which  was  subjected  to  a  shorter 
freezing  period  but  otherwise  treated  the  same,  would  indicate  that 
some  other  factor  than  age  affected  the  yields.  As  a  test  of  age,  there- 
fore, no  significance  can  be  attached  to  the  differences  in  yield.  This  is 
substantiated  by  comparing  the  yield  of  Lot  15  with  the  yields  from 
Lots  12  and  13,  all  of  which  were  of  the  same  kind  and  age.  The  fact 
that  Lot  15  produced  about  two  and  one-half  times  as  much  as  either 
of  the  other  lots  would  indicate  that  in  the  first  four  lots  either  the  rest 
period  was  too  short  or  that  the  roots  were  not  sufficiently  frozen. 
Whichever  the  cause,  the  results  for  this  season  must  be  disregarded  as 
a  test  of  the  best  age  of  root  to  use  for  forcing. 


1927] 


WINTER  FORCING  OF  RHUBARB 


71 


Yields  Increase  With  Age  Up  to  Three  Years 

In  planning  the  experiments  for  the  third  season,  the  variable  fac- 
tors entering  into  the  previous  year's  experiments  were  taken  into  ac- 
count and  included  in  additional  comparisons. 

By  selecting  the  plants  early  in  October,  before  the  tops  died  in 
the  field,  it  was  possible  to  select  those  that  had  produced  large  stalks 
of  deep  red  color.  On  November  21  the  roots  of  these  plants  were  dug. 
One-,  two-,  three-,  and  seven-year-old  roots  were  used.  Typical  one-, 
two-,  and  three-year  roots  are  shown  in  Fig.  6. 

Five  clumps  of  each  age  were  placed  in  cold  storage  at  —10°  F. 
for  five  days.  After  being  thawed  out  slowly,  these  roots  were  bedded 
in  adjacent  sections  in  the  forcing  cellar  on  December  17,  and  desig- 
nated as  Lots  16,  17,  18,  and  19  (Table  8) . 


TABLE  8. — YIELDS  OF  FORCED  RHUBARB  FROM  ROOTS  OF  DIFFERENT  AGES  AND 

WITH  VARYING  REST  PERIODS 
(Five  roots  in  each  lot.   Forced  at  approximately  58°  F.) 


Lot 

Age  of 
roots 

Length  of 
freezing 

Time  of 
growth 

First 
picking 

Last 
picking 

Total 
yield 

Yield 
per   root 

Roots  forced  after  brief  rest  and  freezing.    Bedded  December  17 


days 

Ibs.    oz. 

Ibs. 

16 

1  year 

5  days  at  10°  F. 

41 

Jan.  27 

Feb.  28 

10—  3 

2.03 

17 

2  years 

5  days  at  10°  F. 

38 

Jan.  24 

Feb.  28 

24—  7 

4.9 

18 

3  years 

5  'days  at  10°  F. 

41 

Jan.  27 

Feb.  28 

9—13 

1.95 

19 

7  years 

2  weeks  at-10°F 

47 

Feb.  5 

Feb.  28 

5—  8 

1.1 

Roots  forced  after  six  weeks'  rest  period.   Bedded  January  17 


20 

1  year 

6  weeks  outdoors 

28 

Feb.  14 

Mar.  25 

30—10 

6.2 

21 

2  years 

6  weeks  outdoors 

30 

Feb.  16 

Apr.  4 

38—14 

7.8 

22 

3  years 

6  weeks  outdoors 

30 

Feb.  16 

Apr.  4 

47—10 

9.75 

23 

7  years 

6  weeks  outdoors 

28 

Feb.  14 

Apr.  4 

45—15 

9.2 

Five  clumps  of  each  age  were  left  outdoors  exposed  to  freezing 
temperatures  for  about  six  weeks,  during  which  time  the  temperature 
went  below  zero  several  times,  the  minimum  reached  being  —  20°. 
On  January  15,  roots  of  this  group  were  bedded  in  adjacent  sections  in 
the  forcing  cellar.  These  are  designated  as  Lots  20,  21,  22,  and  23. 

As  in  the  previous  season,  the  roots  that  were  forced  after  a  very 
brief  rest  period  (Lots  16,  17,  18,  and  19)  produced  low  yields.  It 
would  seem  that  the  lack  of  a  sufficient  rest  period  influenced  the 
yields  to  such  an  extent  that  it  overshadowed  the  effect  of  differences 
due  to  age  of  root. 

The  results  from  the  series  that  had  sufficient  rest  period  (Lots 
20,  21,  22,  and  23)  probably  give  the  most  accurate  index  of  differ- 
ences in  yields  due  to  differences  in  age  of  roots.  The  one-year  roots 


72  BULLETIN  No.  298  [October, 

in  this  group  produced  the  smallest  yields,  the  yields  increasing  with 
the  age  of  the  roots  up  to  three  years.  The  seven-year  roots  produced 
practically  the  same  as  the  three-year  roots.  Observations  by  the 
author,  of  a  large  number  of  roots  grown  under  various  conditions  and 
seasons,  bear  out  this  experience.  Two-  and  three-year  roots  pro- 
duce the  largest  stalks  and  the  most  vigorous  growth,  and  it  is  doubt- 
ful if  older  roots  produce  appreciably  larger  yields.  As  the  plants  be- 
come older,  the  stalks  become  smaller  and  often  more  numerous. 

Altho  accurate  color  records  were  not  kept  of  these  lots,  the  one- 
year-old  roots  produced  stalks  that  were  appreciably  lighter  in  color 
than  those  produced  by  older  roots,  similar  to  results  secured  with 
Lots  8  and  9. 


FIG.  6. — TYPICAL  ONE,  Two,  AND  THREE- YEAR-OLD  ROOTS  USED  IN  LOTS 

20,  21,  AND  22 

When  grown  exclusively  for  forcing,  two-year-old  roots  are  best, 
considering  yields  and  costs  of  production.  When  forcing  is  done  as  a 
supplement  to  field  culture,  the  aged  roots  (six  or  seven  years  old) 
which  are  available  will  give  large  profitable  yields  of  well-colored 
stalks. 

The  larger  yields  and  darker  colored  stalks  produced  by  two-year- 
old  roots  as  compared  with  one-year-old  roots  would  make  the  two- 
year  roots  preferable  for  commercial  forcing.  However,  one  has  to 
consider  the  expense  of  caring  for  the  roots  another  year  in  the  field, 
the  slower  turnover  of  capital,  the  charge  for  the  use  of  land  another 
year,  the  increased  labor  of  digging  the  larger  roots,  and  the  larger 
space  required  in  the  forcing  house. 

To  counteract  the  additional  expense  of  producing  the  two-year- 
old  roots,  some  commercial  growers  harvest  a  light  crop  from  these 
roots  in  the  field  at  the  beginning  of  their  second  year.  This  is  gen- 
erally limited  to  one  or  two  pullings  as  soon  as  the  stalks  have  reached 
marketable  size.  At  this  time  the  price  is  high,  and  a  light  harvest  will 
pay  the  expense  of  caring  for  the  crop  for  that  year.  A  heavy  pulling 
should  not  be  made,  as  this  would  seriously  weaken  the  roots  and  re- 
duce the  yield  of  forced  rhubarb.  After  the  second  year  a  good  crop 


1927]  WINTER  FORCING  OF  RHUBARB  73 

may  be  harvested  each  spring  from  the  rhubarb  plantation,  and  this 
will  more  than  offset  the  additional  year's  expense  for  the  roots.  Con- 
sequently there  is  no  additional  cost  for  roots  over  two  years  old. 
Furthermore,  after  a  plantation  has  reached  full  bearing  age  (three 
years)  it  is  generally  more  profitable  not  to  disturb  it  or  use  the  roots 
for  forcing  until  the  plantation  needs  to  be  renewed.  After  five  or  six 
years,  however,  the  yields  will  decrease  and  the  roots  will  have  to  be 
divided  and  replanted  to  renew  the  vigor  of  the  plants.  This  leaves 
a  surplus  of  old  roots,  which  can  be  used  profitably  for  forcing. 

If  roots  are  grown  principally  for  forcing,  there  would  be  no  need 
of  growing  the  roots  more  than  two  years.  It  would  seem  from  these 
experiments  that  two-year-old  roots  are  preferable  to  one-year-old 
roots  for  this  purpose.  This  is  also  in  accordance  with  the  practice  of 
many  commercial  growers.  However,  in  cases  where  a  good  market  for 
outdoor  rhubarb  is  available,  the  most  profitable  practice  would  be  to 
use  old  roots  (six  or  seven  years  old)  for  forcing.  These  would  be 
obtained  from  the  surplus  when  the  old  plantation  was  renewed  by 
dividing  and  replanting  the  divisions  of  the  old  roots.  By  renewing  a 
portion  of  the  plantation  each  year  there  would  be  a  constant  supply 
of  large  old  roots  for  forcing. 

EFFECT  OF  INTENSITY  AND   LENGTH  OF  FREEZING  IN 
BREAKING  REST  PERIOD  AND  INCREASING  YIELD 

It  is  a  well  known  fact  that  rhubarb  cannot  be  forced  successfully 
until  the  roots  have  first  been  thoroly  frozen.  Apparently  freezing 
causes  certain  physiological  changes  and  breaks  the  rest  period,  as  a 
result  of  which  the  roots  are  stimulated  to  a  vigorous  growth  when 
given  favorable  conditions  of  temperature  and  moisture. 

That  the  length  of  the  dormant  period  before  forcing  might  also 
be  an  important  factor  affecting  the  yield  of  forced  rhubarb  was  sug- 
gested by  the  striking  differences  in  the  yields  of  Lots  12  and  15,  Table 
7,  and  the  low  yields  of  Lots  10,  11,  12,  and  13,  Table  7,  compared  with 
those  from  Lots  1,  2,  and  3,  Table  1,  and  from  Lots  8  and  9,  Table  5. 

The  experiments  for  the  third  season  (1924-25)  therefore  were  so 
planned  that  the  effects  of  these  factors  might  be  determined. 

From  a  lot  of  selected  seven-year-old  roots  of  the  same  variety 
and  strain,  that  had  been  grown  under  identical  conditions,  twenty- 
five  clumps  were  selected  of  uniform  size.  These  were  dug  on  Novem- 
ber 21.  They  were  divided  into  five  lots  of  five  clumps  each.  These 
lots  were  placed  in  cold  storage  on  December  6. 

Lots  25  and  28  were  placed  in  a  temperature  of  25°  F.  It  was 
thought  that  this  temperature  would  freeze  the  roots  lightly.  However, 
after  72  hours  at  this  temperature  the  roots  remained  unfrozen.  Con- 
sequently the  temperature  was  reduced  on  December  9  to  20°  F.  On 
December  10  this  storage  chamber  was  not  properly  regulated  and  the 


74 


BULLETIN  No.  298 


[October, 


temperature  dropped  to  2°  F.,  but  within  24  hours  was  back  to  20°  F. 
and  was  kept  constant  at  this  temperature  for  the  remainder  of  the 
cold  storage  period. 

Lots  26  and  29  were  subjected  to  a  temperature  of  10°  F.  thru- 
out  the  cold  storage  period. 

Lot  30  was  subjected  to  a  temperature  of  —10°  F.  thruout  the 
cold  storage  period.  Lot  19,  described  in  the  previous  test,  is  brought 
into  the  comparison  at  this  point  as  a  companion  to  Lot  30,  since  it 
had  the  same  history  and  treatment  with  the  exception  of  a  longer 
rest  period. 

On  account  of  the  unintentional  lowering  of  the  temperatures  in 
Lots  25  and  28,  two  other  lots  of  the  same  kind  of  roots  were  placed 
in  cold  storage  on  December  12,  1924.  These  were  designated  as  Lots 
24  and  27  and  were  subjected  to  a  constant  temperature  of  20°  F. 
thruout  the  cold  storage  period. 


TABLE  9. — EFFECT  OF  INTENSITY  OF  FREEZING  AND  LENGTH  OF  REST  PERIOD  UPON 

YIELDS  OF  FORCED  RHUBARB 
.  (Five  roots  in  each  lot,  seven  years  old.    Forced  at  approximately  58°  F.) 


Lot 

Length  and  severity  of 
freezing 

Time  of 
growth 

First 
picking 

Last 
picking 

Total 
yield 

Yield 
per  root 

Roots  forced  immediately  after  freezing.  Bedded  December  17 


24 

2  weeks  at  20°  F  

days 
33 

Jan.  27s 

Mar.  25 

Ibs.  oz. 
38—  2 

Ibs. 
7.62 

25 

2  weeks  at  20°  F1  

38 

Jan.  24 

Mar.  18 

33—12 

6  75 

26 

2  weeks  at  10°  F  

38 

Jan.  24 

Mar.  18 

28—  4 

5.65 

19 

2  weeks  at-  10°  F  

47 

Feb.  5 

Feb.  28 

5—  8 

1.10 

Additional  rest  period  at  31°  for  1  month  before  forcing.    Bedded  January  17 


27 

2  weeks  at  20°  F  

26 

Feb.  12 

Apr.  17 

65—10 

13.12 

28 

2  weeks  at  20°  F1  

26 

Feb.  12 

Apr.  4 

63—10 

12.68 

29 

2  weeks  at  10°  F  

26 

Feb.  12 

Apr.  4 

75 

15.00 

30 

2  weeks  at  —  10°  F  

26 

Feb.  12 

Apr.  4 

16—  7 

3.3 

23 

6  weeks  outdoors  

28 

Feb.  14 

Apr.  4 

45—15 

9.2 

temperature  in  this  chamber  dropped  to  2°  F.  on  December  10,  as  a  result 
of  improper  regulation,  but  within  24  hours  was  back  again  to  20°  F. 
2Bedded  one  week  later. 

After  two  weeks  in  the  freezing  chambers,  Lots  24,  25,  26,  and  19 
were  removed  from  their  freezing  chambers  and  bedded  in  adjacent 
sections  of  the  forcing  cellar  on  December  17.  At  the  time  these  lots 
were  removed  from  their  freezing  chambers  their  companion  lots  (Lots 
27,  28,  29,  and  30)  were  likewise  removed  from  their  freezing  cham- 
bers and  were  placed  in  cold  storage  at  31°  F.  for  one  month.  At  this 
temperature  they  thawed  out  but  remained  dormant.  After  this  addi- 
tional rest  period,  these  lots  were  bedded  on  January  17  in  the  same 
forcing  cellar  with  their  companion  lots  and  forced  under  similar  con- 
ditions. (Table  9) 


1927]  WINTER  FORCING  OF  RHUBARB  75 

Lot  23  also  is  brought  into  this  comparison,  since  it  was  of  the 
same  age  and  received  the  same  treatment  except  that  it  had  been  left 
outdoors  to  freeze  during  this  six  weeks'  period.  During  this  time  the 
temperature  went  below  zero  on  several  occasions,  the  minimum  being 
_  20o  p  This  lot  was  likewise  bedded  in  the  forcing  cellar  on  Jan- 
uary 17. 

Any  differences  in  the  yields  of  Lots  24,  25,  26,  and  19  may  reas- 
onably be  attributed  to  the  effect  of  different  freezing  temperatures, 
that  being  the  only  essential  difference  in  their  treatment.  Differences 
among  Lots  27,  28,  29,  and  30  likewise  may  be  attributed  to  this  fac- 
tor, since  it  was  the  only  essential  one  in  which  they  varied. 

In  both  these  series  a  light  freezing,  two  weeks  at  20°  F.  (Lots 
24  and  27),  appeared  to  be  sufficient  to  break  the  rest  period.  This  is 
shown  by  the  prompt  growth  in  comparison  with  the  unfrozen  roots 
of  Lot  14  in  Table  7.  Furthermore  very  severe  freezing,  two  weeks  at 
-  10°  F.  (Lots  19  and  30) ,  seemed  to  injure  the  roots,  for  in  each  case 
the  yields  decreased  where  the  freezing  was  more  intense. 

Altho  the  light  freezing  was  sufficient  to  cause  the  physiological 
changes  resulting  in  prompt  growth  under  forcing  conditions,  large 
yields  were  not  obtained  from  any  roots,  regardless  of  the  intensity  of 
freezing,  which  were  allowed  only  a  short  rest  period.  This  suggested 
that  the  length  of  rest  period  might  be  a  factor  affecting  yields. 

LENGTH  OF  REST  PERIOD  FOR  MAXIMUM  YIELDS 
An  examination  of  Tables  8  and  9  shows  eight  direct  comparisons 
in  which  length  of  rest  period  was  the  only  variable  factor  (in  Table 
8  compare  Lot  16  with  Lot  20,  17  with  21,  18  with  22,  and  19  with  23; 
in  Table  9  compare  Lot  24  with  Lot  27,  25  with  28,  26  with  29,  and 
19  with  30) .  In  every  case  the  yields  were  increased  from  two  to  three- 
fold and  the  resulting  growth  was  more  rapid  where  the  roots  had  a 
longer  rest  period.  The  results  are  conclusive  in  showing  that  the 
length  of  the  rest  period  is  a  most  important  factor  affecting  yields  and 
rapidity  of  growth ;  and  that  a  rest  period  of  at  least  six  or  eight  weeks 
is  essential  to  maximum  yields. 

Apparently  a  rest  period  of  this  length  is  necessary  for  certain 
physiological  changes  to  take  place  in  the  roots  before  maximum 
growth  will  result.  Altho  freezing  is  necessary  to  break  this  rest  period 
(Lot  14,  Table  7),  a  light  freezing  is  sufficient  to  break  it  so  that 
growth  will  start  promptly  (Lots  24  and  27,  Table  9).  Severe  freezing 
is  only  injurious  and  is  not  effective  in  reducing  the  rest  period  needed 
for  maximum  growth  (Lots  25,  26,  and  19,  and  28,  29,  and  30,  Table 
9). 


76  BULLETIN  No.  298 

CONCLUSIONS 

1.  Temperature  is  an  important  factor  affecting  both  the  yield 
and  color  of  forced  rhubarb.   The  lower  the  forcing  temperature,  the 
slower  the  growth  and  the  more  red  or  dark  pink  color  in  the  stalks. 
Slow  growing,  dark  red  stalks  can  be  produced  at  a  forcing  tempera- 
ture of  50°  F.    Taking  into  consideration  yield,  color  and  rate  of 
growth,  the  optimum  forcing  temperature  is  about  59°  F.  At  this  tem- 
perature large  yields  of  well-colored  stalks  can  be  produced.  A  quicker 
crop  can  be  grown  at  68°  F.  but  the  color  will  be  inferior.    Seventy- 
seven  degrees  F.  is  too  high  for  healthy  growth. 

2.  Watering  the  roots  abundantly  during  the  forcing  period  greatly 
increases  the  yield  and  does  not  reduce  the  red  color  of  the  stalks. 

3.  Rhubarb  roots  one-year  old  or  older  can  be  used  for  forcing. 
One-year-old  roots  produce  the  lightest  colored  stalks  and  smallest 
yields.    Yields  increase  with  the  age  of  the  roots  up  to  three  years. 
When  roots  are  to  be  grown  exclusively  for  forcing,  one-year  or  two- 
year  roots  are  best,  the  latter  having  the  advantages  of  heavier  yield 
and  better  color  to  offset  the  increased  cost  of  the  roots.   Aged  roots 
(five  to  seven  years  old)  which  are  available  whenever  a  plantation  is 
replanted,  produce  large  profitable  yields  of  well-colored  stalks. 

4.  Freezing  the  roots,  thus  causing  certain  physiological  changes 
that  stimulate  growth,  is  essential  before  rhubarb  can  be  forced  profit- 
ably. 

5.  A  light  freezing  (20°  F.  for  two  weeks)  is  sufficient  to  cause 
the  physiological  changes  which  stimulate  growth.   The  roots  do  not 
freeze  at  25°  F. 

6.  Severe  freezing  ( —  10°  F.)  is  injurious,  reduces  the  yield,  and 
does  not  shorten  the  rest  period  needed  for  maximum  yields. 

7.  A  brief,  thoro  freezing  (two  weeks  at. 20°  F.)  followed  by  a 
dormant  period  (an  additional  four  weeks  at  temperature  too  low  for 
growth)  is  as  effective  in  stimulating  growth  as  continuous  freezing 
thruout  the  same  period. 

8.  The  length  of  the  rest  period  before  forcing  is  an  important 
factor  affecting  yields.    In  general,  the  longer  the  rest  period,  the 
greater  the  subsequent  yield  and  the  more  rapid  the  growth.  A  rest 
period  of  at  least  six  or  eight  weeks  is  essential  for  maximum  yields. 


UNIVERSITY  OF  ILLINOIS-URBAN* 


